I'd also assume, if smithing rules are the same as goldsmithing rules, that 6 iron nuggets + 1 iron ore would produce an iron ingot and the cap for that would be 2 levels above the hard cap for iron ingots (20).

I was alchemy 88 when I discovered this, and in 4 stacks of lightning crystals I got 5 stacks of iron nuggets (it was full moon 90% lightsday). If you're looking for iron nuggets for some reason, I suggest finding a different method of obtaining these because mamushtos are a level 91 alchemy synthesis, cost about 5-7k to make depending on your sever, and these nuggets should only be worth 1k/stack tops judging by ore prices from npcs.